Bone defects and fractures have spurred widespread research into repairing and regenerating tissue and bone formation. Many synthetic bone graft substitutes have been developed which provide suitable osteoconductivity. See Calori G M, Mazza E, Colombo M, Ripamonti C. The use of bone-graft substitutes in large bone defects: any specific needs? Injury 2011; 42 Suppl 2:S56-63; Greenwald a S, Boden S D, Goldberg V M, Khan Y, Laurencin C T, Rosier R N. Bone-graft substitutes: facts, fictions, and applications. J Bone Joint Surg Am 2001; 83-A Suppl:98-103; and Zimmermann G, Moghaddam A. Allograft bone matrix versus synthetic bone graft substitutes. Injury 2011; 42 Suppl 2:S16-21. However, improvements in the osteoinductivity and osteogenicity of synthetic bone graft substitutes could provide more rapid tissue repair and, consequently, an increase in the number of successful unions. See Giannoudis P V, Calori G M, Begue T, Schmidmaier G. Bone regeneration strategies: current trends but what the future holds? Injury 2013; 44 Suppl 1:S1-2; and Carson J S, Bostrom M P G. Synthetic bone scaffolds and fracture repair. Injury 2007; 38 Suppl 1:S33-7.
The osteoinductivity and osteogenicity of bone tissue engineering substitutes could be improved by the delivery of appropriate growth factors to healing tissue along biologically relevant timelines. See Lauzon M-A, Bergeron E, Marcos B, Faucheux N. Bone repair: new developments in growth factor delivery systems and their mathematical modeling. J Control Release 2012; 162:502-20. While several studies have indicated that the delivery of growth factors, the conjugation of growth factors, prolonged growth factor release, and the delivery of more than one growth factor, may under certain conditions hasten and enhance bone tissue repair, none have achieved sequential and sustained release of growth factors along biologically relevant timelines. See Kempen D H R, Lu L, Heijink A, Hefferan T E, Creemers L B, Maran A, et al. Effect of local sequential VEGF and BMP-2 delivery on ectopic and orthotopic bone regeneration. Biomaterials 2009; 30:2816-25; Boerckel J D, Kolambkar Y M, Dupont K M, Uhrig B, Phelps E, Stevens H Y, et al. Effects of protein dose and delivery system on BMP-mediated bone regeneration. Biomaterials 2011; 32:5241-51; Chung Y-I, Ahn K-M, Jeon S-H, Lee S-Y, Lee J-H, Tae G. Enhanced bone regeneration with BMP-2 loaded functional nanoparticle-hydrogel complex. J Control Release 2007; 121:91-9; Geuze R, Theyse L. A Differential Effect of BMP-2 and VEGF Release Timing on Osteogenesis at Ectopic and Orthotopic Sites in a Large Animal Model. Tissue Eng Part A 2012:1-34; and Ferrara N. VEGF and the quest for tumour angiogenesis factors. Nat Rev Cancer 2002; 2:795-803.
There is a need to improve the rate of non-union in long bone fractures. Estimates indicate that 5-10% of bone fractures exhibit impaired healing, either delayed unions or non-unions, and that many of these result from critical size defects. See Calori G M, Mazza E, Colombo M, Ripamonti C. The use of bone-graft substitutes in large bone defects: any specific needs? Injury 2011; 42 Suppl 2:S56-63; Frölke J, Patka P. Definition and classification of fracture non-unions. Injury 2007; 38S:S19-22; Tzioupis C, Giannoudis P. Prevalence of long-bone non-unions. Injury 2007; 44; and Calori G, Albisetti W, Agus A, Iori S, Tagliabue L. Risk factors contributing to fracture non-unions. Injury 2007:S11-S18.
Limitations in utilizing autograft and allograft sources to promote bone repair and regeneration have led to extensive research in synthetic bone graft substitutes and bone tissue engineering scaffolds aimed at preserving the advantages of autografting and allografting while overcoming their limitations and constraints. See Zimmermann G, Moghaddam A. Allograft bone matrix versus synthetic bone graft substitutes. Injury 2011; 42 Suppl 2:S16-21; O'Brien F J. Biomaterials & scaffolds for tissue engineering. Mater Today 2011; 14:88-95; Salgado A J, Coutinho O P, Reis R L. Bone tissue engineering: state of the art and future trends. Macromol Biosci 2004; 4:743-65; and Szpalski C, Wetterau M, Barr J, Warren S M. Bone Tissue Engineering: Current Strategies and Techniques—Part I: Scaffolds 2012; 18.
Numerous patent applications and papers have been published in this field. See U.S. patent application Ser. No. 12/039,666; WO 2011/109834; U.S. Pat. No. 6,296,667; WO 2012/118843; U.S. Pat. No. 8,022,040; U.S. patent application Ser. No. 13/435,259; U.S. Pat. No. 7,758,882; Kempen, D. H., et al. 2009. Effect of local sequential VEGF and BMP-2 delivery on ectopic and orthotopic bone regeneration. Biomaterials, 30: 2816-2825; U.S. Pat. No. 8,328,876; U.S. Pat. No. 7,163,691; U.S. Pat. No. 8,318,674; U.S. Pat. No. 8,309,518; U.S. Pat. No. 8,303,973; US Pat. No. 8,293,486; and EP2300033.
Current methods for treating bone defects and fractures via the delivery of VEGF and BMP-2 do not sufficiently achieve sustained release of the growth factors along the physiologically-relevant timelines necessary to enhance spatio-temporal growth factor expression. This is because conventional methods rely on passive adsorption of growth factors on the scaffold biomaterial surface or sponge-like absorption of a growth factor-containing solution within scaffold pore spaces. The conventional methods release a single, super-physiological bolus dose of growth factors, which delivers an excess of growth factors earlier than needed for optimum repair and/or regeneration of tissue and/or bone. Further, conventional methods do not allow for coordinated, sequential delivery of growth factors within the fracture gap and thus cannot take advantage of synergistic effects that might arise from sequential delivery of complimentary growth factors. Moreover, clinical, passive adsorption of BMP-2 in spinal fusion applications has not only resulted in a lack of temporal control of BMP-2 release, but also a lack of spatial control and delivery has been seen. Occasionally the conventional mechanisms have been implicated in negative outcomes including extradiscal, ectopic, or heterotopic ossification and an associated risk of edema.
Therefore, notwithstanding the advancements in the field, many limitations remain in current treatments that contribute to impaired healing, delayed bony unions and non-unions. Thus, there exists a need for improved synthetic bone graft substitutes. The invention described herein provides such an article and methods of using it. The synthetic bone graft substitutes described herein enable sequential and sustained release of growth factors and provide a powerful tool to promote and expedite successful bony union in otherwise problematic fracture non-unions and bone defects.